How to adjust the Responsiveness of a Value?

Question

I have a "value" of a "form" that has a text "Say hello".

I need to increase the font-size that is there. How I do?

• HTML5:

          <section class="block-body">
              <div class="row">
                  <div class="col-md-6 col-md-offset-3">
                      <form class="form" role="form">
                          <div class="form-group">
                              <input type="text" class="form-control form-control-white" id="subject" placeholder="Seu Nome" required>
                          </div>
                          <div class="form-group">
                              <input type="email" class="form-control form-control-white" id="exampleInputEmail2" placeholder="Email" required>
                          </div>
                          <div class="form-group">
                              <input type="text" class="form-control form-control-white" id="subject" placeholder="Empresa" required>
                          </div>
                          <div class="form-group">
                              <input type="tel" class="form-control form-control-white" id="subject" placeholder="Contato" required>
                          </div>
                          <div class="form-group">
                              <input type="text" class="form-control form-control-white" id="subject" placeholder="Assunto" required>
                          </div>
                          <div class="form-group">
                              <textarea class="form-control form-control-white" placeholder="Mensagem" required></textarea>
                          </div>
                        <input type="submit" class="btn btn-o-white" value="Diga Olá">
                      </form>
                  </div>
              </div>
          </section>
  

Answer 1

Only increases the font in css!

<style>
.btn-o-white{
    font-size: 32px;
}
</style>
<input type="submit" class="btn btn-o-white" value="Diga Olá">